1. Field of the Invention
The present invention relates to an alignment correction method of adjusting plane-directional relative positions of a plurality of patterns which are formed in fabrication of semiconductor devices, and a semiconductor device fabricated through the alignment method.
2. Description of the Background Art
Alignment is described in detail with reference to a conceptual diagram shown in FIG. 13. Planes 3a and 3b include patterns 1a and alignment marks 2a to 2d and patterns 1b and alignment marks 2e to 2h respectively. The patterns 1a and 1b, which are formed on wafers respectively, are made of a silicon compound, a metal or the like. The alignment marks 2a to 2d are formed simultaneously with the patterns 1a, while the alignment marks 2e to 2h are formed simultaneously with the patterns 1b. In a process of fabricating semiconductor devices, the patterns 1a must be stacked on correct positions on the patterns 1b. The alignment is adapted to relatively align the positions of such patterns with each other. Among processes of fabricating semiconductor devices, an exposure process requires such alignment. In the exposure process, reticles are aligned with wafers in practice.
FIG. 14 is a block diagram showing a conventional production management system for managing fabrication of semiconductor devices. Numerals 4a, 4b, . . . denote stepping projection aligners (hereinafter referred to as steppers) employed in the aforementioned exposure process, numerals 5a, 5b, . . . denote overlay checking devices, numeral 6 denotes a production management system body, numeral 61a denotes an alignment correction part, numeral 6b denotes a data base, numerals 7, . . . denote semiconductor fabrication devices such as sputtering devices or etching devices, and numeral 8 denotes a reference terminal for referring to the contents of the data base 6b.
A shift amount is caused between patterns which are aligned with each other by the steppers despite the alignment, due to various causes such as mechanical errors of the steppers 4a, 4b, . . . themselves and the like.
A correction value (hereinafter referred to as stepper correction value) for eliminating such shift amount is set in the steppers. On the other hand, the overlay checking device detects the shift amount and calculate correction value (hereinafter referred to as overlay check correction value) for eliminating the shift amount.
The production management system body 6 manages data (alignment data) related to the alignment, i.e., the overlay check correction value, the stepper correction values, wafer types such as lot numbers, product numbers and the like, the date of the alignment, contents of processing, the production history and the like. The alignment data are stored in the data base 6b.
The alignment correction part 61a, which is a function of the production management system body 6, calculates the stepper correction values.
A conventional alignment correction method which is carried out by the production management system shown in FIG. 14 is now described with reference to a flow chart shown in FIG. 15. First, the production management system body 6 sets the stepper correction value (step 901). Then, the exposure process is carried out, while the stepper 4a performs alignment (step 902). Then, the overlay checking device 5a detects a shift amount between alignment marks (step 903). Then, the overlay checking device 5a calculates overlay correction value from the detected shift amount (step 904). Then, the alignment correction part 61a calculates a stepper correction value to be set in next alignment from the overlay check correction value calculated at the step 904 and the alignment data managed by the production management system body 6 (step 905). Then, the production management system body 6 adds the alignment data related to the steps 901 to 904 to the data base 6b and manages the same (step 906). This also applies to the remaining steppers 4b, . . . and the remaining overlay checking devices 5b, . . .
A method of forming the stepper correction value set by the alignment correction part 61a in the steppers 4a, 4b, . . . is now described. First, a true shift amount is defined. The true shift amount is expressed as follows:
true shift amount=stepper correction value-overlay check correction value . . . (equation 1)
The stepper correction value and the overlay check correction value appearing on the right side of the equation 1 are those set in each stepper and calculated by each overlay checking device 5 at the steps 901 and 905 in FIG. 15 respectively. Namely, the true shift amount is actual shift amount resulting from alignment performed by the steppers 4a, 4b, . . . This is now described in detail with reference to a one-directional shift amount in alignment offsets x and y illustrated in FIG. 16. As shown in FIG. 16, it is assumed that a stepper correction value for correcting a shift amount of +1 is set, and a overlay check correction value for correcting a shift amount of -1 results after alignment. The difference of +2 between the stepper correction value and the overlay check correction value is the true shift amount. Assuming that a true shift amount of +2 results also after next alignment, therefore, it comes to that a overlay check correction value is zero, i.e., no shift amount is caused when the true shift amount of +2 is set as a stepper correction value for the next alignment.
In practice, however, true shift amounts are not necessarily regularly constant, due to various causes such as mechanical errors of the steppers 4a, 4b, . . . themselves. Therefore, it is necessary to predict true shift amount resulting after the next alignment. The alignment correction part 61a performs this prediction.
A method of predicting true shift amounts carried out by the alignment correction part 61a is now described. The production management system carries out various processes for fabricating semiconductor devices with the steppers 4a, 4b, . . . and the semiconductor fabrication devices 7, . . . Every time alignment is performed through the steppers 4a, 4b, . . . the production management system body 6 stores alignment data in the data base 6b and manages the same. The alignment data include true shift amounts, which are the differences between stepper correction values and overlay check correction values. Among the true shift amounts stored in the data base 6b, those satisfying conditions (alignment conditions) for the next alignment are extracted. The conditions for the next alignment are as follows: An exposure processing time is within a specific range from the next alignment, and wafer type, process code, and stepper code of stepper employed for forming upper and lower layer patterns respectively are identical to those for the next alignment. The aforementioned specific range is previously set by an operator. Assuming that the pattern 1a shown in FIG. 13 is upper layer pattern formed in the next alignment, the previously formed pattern 1b corresponds to lower layer pattern, for example.
FIG. 18 is a graph showing trend (fluctuations with respect to processing times) of the extracted true shift amounts. Referring to FIG. 18, the horizontal axis shows dates of alignment. Each recording point shown in this graph satisfying the aforementioned alignment conditions is recorded every time patterns on each wafer are changed, i.e., every alignment. This is because the shift amounts as well as the correction values are changed if the patterns are changed, due to influences on optical measurement for the alignment.
The alignment correction part 61a predicts true shift amount in the next alignment (date tx) from this trend. This trend is employed since the true shift amounts in the next alignment can be correctly predicted by employing the trends in the same alignment conditions as those for the next alignment.
The alignment correction part 61a predicts an average value of recent three recording points P1, P2 and P3 as a true shift amount in the next alignment, as shown in FIG. 18. Further, the alignment correction part 61a forms the predicted true shift amount as a stepper correction value which is set in the stepper 4a in the next alignment.
Thus, the method of predicting true shift amount carried out by the alignment correction part 61a is adapted to calculate stepper correction value which is set in the steppers in the next alignment only from the trends of the true shift amounts related to the same alignment conditions as those for the next alignment.
The conventional alignment correction method of the aforementioned structure has the following problems (1) to (3): (1) Referring to FIG. 18, influence by fluctuation of the trend is relatively small if the time between a date t1 of precedent alignment and the date tx is relatively short, while the influences may be so increased that the stepper correction values set in the next alignment may be improper if the time is relatively long. (2) FIG. 19 is a graph showing exemplary trends of correction values with respect to dates. The true shift amounts are predicted since the same are not necessarily regularly constant, as described above. As shown in FIG. 19, however, true shift amount which is the difference between stepper correction value D1 and overlay check correction value D2 may be substantially constant. If the true shift amounts are constant, it means that shift amounts result by fluctuations of the overlay check correction values. If the true shift amounts are substantially constant, therefore, the stepper correction values may be improper. (3) If shift amount detected by any overlay checking device 5 is out of a determined standard range (hereinafter referred to as nonstandardization), the cause for the nonstandardization cannot be determined.